Automatic transmissions are known in which planetary gearing is used and in which gear shifts are effected by the activation of couplings or brake bands (servo-elements) which either range or disengage the annulus, the cage carrying the planet gears or the sun gear. The couplings and brake bands are generally operated hydraulically. The pressure required herefor is furnished by a primary pump which also furnishes the pressure for other hydraulic elements such as a torque converter, the servo-controls, etc. The vehicle in which the automatic transmission is installed generally has a mode selector lever in which a particular one of a plurality of modes, such as P, N, D, 1, 2, or R may be selected by the driver. The main pressure is applied to this lever through a servo-valve. It is also applied to the gear shift valves. The gear shift valves are operated in accordance with the operating conditions of the vehicle and the position of the mode selector lever and cause the main pressure to be applied through throttling elements to the couplings of the transmission, thereby assuring a soft transition.
The known systems of the above described type have the disadvantage that they require a great deal of equipment such as gear shift valves and throttling elements independent of whether the shift points are determined hydraulically or electronically.